(1) Field of the Invention
The present invention relates to a radio transmission apparatus and a method of inserting a guard interval. The present invention relates to a technique suitable for use in, for example, a communication scheme in which a guard interval is added to a transmission data block.
(2) Description of Related Art
For a radio access scheme for next-generation mobile communications, a transmission scheme, represented by OFDM (Orthogonal Frequency Division Multiplexing), in which a guard interval (GI) such as a cyclic prefix is added to a signal and signal processing is performed in a frequency domain is considered. A well-known feature of the transmission scheme is a high tolerance to a radio channel in a strong frequency-selective wideband.
Now, the principle of the OFDM scheme will be described.
FIG. 15 is a block diagram showing an exemplary configuration that focuses attention on an essential part of a radio transmission apparatus that adopts the OFDM scheme. A radio transmission apparatus 100 shown in FIG. 15 can be applied to a transmission system of a base station apparatus (BTS: Base Transceiver Station) that composes a mobile communication system, or a mobile terminal (MS: Mobile Station). The radio transmission apparatus 100 includes, for example, a turbo encoder 101, a data modulator 102, a data/pilot signal multiplexer 103, an IFFT (Inverse Fast Fourier Transformer) 104, a GI insertion unit 105, a D/A (digital/analog) converter 106, an RF transmitter 107, and a transmission antenna 108.
In the radio transmission apparatus (hereinafter also referred to as the “transmission station”) 100 having such a configuration, first, in the turbo encoder 101, turbo encoding which is a type of error-correction encoding is performed on a data signal to be transmitted. In the data modulator 102, data modulation is performed on the data signal using a multivalue orthogonal modulation scheme such as QPSK (Quadrature Phase Shift Keying), 16 QAM (Quadrature Amplitude Modulation), or 64 QAM. Then, in the data/pilot signal multiplexer 103, the modulated data signal is time- or frequency-multiplexed with a pilot signal which is a signal known between the transmission station 100 and a radio reception apparatus (hereinafter also referred to as the “reception station”) 200.
The multiplexed signal is subjected to an IFFT processing on a unit basis of a certain amount of samples (OFDM symbols) in the IFFT 104, whereby the signal is converted from a frequency-domain signal to a time-domain signal. Thereafter, in the GI insertion unit 105, a cyclic prefix is inserted (added), as a GI, in (to) the signal.
Specifically, as shown in FIG. 16, for example, in each OFDM symbol(=NFFT-sample) having been subjected to an IFFT processing, an NCPre-sample at a rear (see a hatched portion 600) is cyclically copied and the copy is inserted (added) as a cyclic prefix 601 in (to) a head of each OFDM symbol, whereby the cyclic prefix 601 acts as a guard interval of each OFDM symbol. Since the cyclic prefix 601 is cyclically copied, in a period of a (NFFT+NCPre)-sample in which the cyclic prefix 601 has been inserted, a signal is continuous.
Then, the GI-inserted signal is D/A converted by the D/A converter 106. Thereafter, in the RF transmitter 107, the converted signal is subjected to necessary radio transmission processings including orthogonal modulation, frequency conversion (up conversion) from a baseband signal to a radio frequency (RF) signal, and the like. The processed signal is then transmitted toward the reception station 200 from the transmission antenna 108.
On the other hand, FIG. 17 is a block diagram showing a configuration that focuses attention on an essential part of a radio reception apparatus 200 that adopts the OFDM scheme. The radio reception apparatus 200 shown in FIG. 17 can be applied to a reception system of a BTS or an MS. The radio reception apparatus 200 includes, for example, a reception antenna 201, an RF receiver 202, an A/D converter 203, an FFT timing detector 204, a GI remover 205, an FFT (Fast Fourier Transformer) 206, a data/pilot signal demultiplexer 207, a channel estimator 208, a channel compensator 209, a data demodulator 210, and a turbo decoder 211.
In the reception station 200 having such a configuration, an RF signal transmitted from the transmission station 100 is received by the reception antenna 201. The received signal is subjected to necessary radio reception processings including frequency conversion (down conversion) to a baseband signal, orthogonal demodulation, and the like, in the RF receiver 202. Then, the processed signal is A/D converted by the A/D converter 203. The converted signal is input to the FFT timing detector 204 and the GI remover 205.
The FFT timing detector 204 computes a correlation between the received signal from the A/D converter 203 and a replica of a transmission pilot signal (pilot replica) and thereby detects reception timing (starting point for an effective signal component) of the received signal (direct wave).
The GI remover 205 removes, based on information on the reception timing detected by the FFT timing detector 204, the cyclic prefixes from the received signal from the A/D converter 203 and cuts out an effective signal component (e.g., an NFFT-sample) of each OFDM symbol.
FIG. 18 shows an example of such an operation. In FIG. 18, for convenience of description, a received signal is represented such that the received signal is separated into components for paths (paths #1 and #2). If an influence of thermal noise is disregarded, for the path #1, it can be seen that only an effective signal component of an OFDM symbol n in which a cyclic prefix 601 is removed is precisely cut out by an NFFT-sample period (also referred to as an “FFT window”). On the other hand, for the path #2, although a signal is cut out (extracted) including part of the cyclic prefix 601, since the cyclic prefix 601 is, as described above, one obtained by cyclically copying an effective signal component of an OFDM symbol, it turns out that only an effective signal component (NFFT-sample) of the OFDM symbol n is precisely cut out. That is, a multipath component whose delay time is within the length of the cyclic prefix 601 (GI) can be received without causing interference between OFDM symbols.
The signal from which the cyclic prefixes 601 have been removed is subjected to an FFT processing in the FFT 206, whereby the signal is converted from a time-domain signal to a frequency-domain signal. Thereafter, in the data/pilot signal demultiplexer 207, the signal is demultiplexed into time- or frequency-multiplexed data and pilot signals. The received pilot signal is input to the channel estimator 208 and the received data signal is input to the channel compensator 209.
The channel estimator 208 computes a correlation between the received pilot signal and a replica of a transmission pilot signal, and thereby estimates channel distortion in a radio channel (obtains a channel estimate). The channel compensator 209 multiplies the received data signal which is demultiplexed by the data/pilot signal demultiplexer 207 by a complex conjugate of the channel estimate obtained by the channel estimator 208, thereby suppressing (compensating) the channel distortion. The received data signal after the channel compensation is subjected to data demodulation in the data demodulator 210. Then the demodulated signal is subjected to a turbo decoding (error-correction decoding) in the turbo decoder 211.
Now, a frequency spectrum of a transmitted signal in the OFDM scheme is considered. FIG. 19 is a diagram showing an exemplary frequency spectrum of a signal transmitted from the transmission station 100 described in FIG. 15, where a horizontal axis represents a frequency normalized by a system bandwidth and a vertical axis represents power (relative power) normalized by transmission power in the vicinity of a center frequency.
In the example shown in FIG. 19, since power is slowly converged outside an effective subcarrier, adjacent band radiation is large. This results from a frame format of the OFDM scheme shown in FIG. 16. Namely, in a signal in which cyclic prefixes 601 have been inserted, inside one OFDM symbol(=NFFT+NCPre-samples), the signal is continuous as described above; however, at a boundary of each OFDM symbol, the signal is discontinuous. This corresponds to an application of a rectangular time-domain window function (hereinafter may be abbreviated as a “time window”) to each OFDM symbol. Thus, in the frequency spectrum, a waveform is such that a Sinc function is convoluted and the convergence of power becomes moderate.
As one of techniques for reducing such adjacent band radiation, there is a known technique in which a time window having a shape other than a rectangular shape is applied so that a signal is moderately attenuated at a boundary of an OFDM symbol. FIG. 20 shows an operation of applying a time window having a shape other than a rectangular shape to a signal in which cyclic prefixes 601 have been inserted.
Specifically, as shown in (1) and (2) of FIG. 20, first, given that a period (window width) where the signal is to be attenuated by a time window is an Nwin-sample, two areas (hatched portions 602 and 603) of each OFDM symbol exclusive of a cyclic prefix 601 (NCPre-sample) are cyclically copied, whereby Nwin/2 samples are obtained. The Nwin/2 samples are inserted in both ends of the OFDM symbol, respectively. Note that in an (NFFT+NCPre+Nwin)-sample period after the insertion, the signal is continuous. Then, as shown in (3) of FIG. 20, a time window is applied to the Nwin-sample periods present at both sides of the (NFFT+NCPre+Nwin)-sample period. Here, as a window function, a raised cosine function is used.
Thereafter, as shown in (4) of FIG. 20, OFDM symbols are connected such that the periods where the signal is to be attenuated by time windows overlap with each other between adjacent OFDM symbols. FIG. 21 shows a frequency spectrum of a transmitted signal for the case in which a time window of a raised cosine function is applied. As shown in FIG. 21, by applying a time window of a raised cosine function, the signal is attenuated in the vicinity of a discontinuous point at an OFDM symbol boundary; thus, it can be seen that the convergence of power is steeper as compared with the case shown in FIG. 19 in which a rectangular time window is applied.
The adjacent band radiation can be reduced not only by using the aforementioned window function but also by using, for example, a band-limiting filter that has steep frequency characteristics.
As other conventional multicarrier transmission techniques using a guard interval, there are techniques proposed by following Patent Documents 1 and 2.
The technique of Patent Document 1 proposes that when a filter having large fluctuations in a rise/fall transient response is used to filter a multicarrier signal, by adding a signal of a time width corresponding to the transient response time to a head and an end of a single-burst multicarrier signal having been inverse discrete Fourier transformed, degradation of transmission characteristics which is caused by transmission waveform distortion resulting from a transient response of the filer is reduced.
The technique of Patent Document 2 relates to a technique for reducing the amount of computation associated with a window function processing for suppression of spurious due to discontinuity between OFDM signal symbols. In a window function processing for preventing discontinuity between data intervals which are temporally cut out in the above manner described above, i.e., in a process in which samples at a head and an end of an OFDM symbol are multiplied by a predetermined weighting factor so that both ends of the symbol smoothly approximate “0”, a multiplier is not required and even when a data transmission rate is increased, it is possible to avoid increase in cost and power consumption in a radio transmission apparatus.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2001-156740
[Patent Document 2] Japanese Patent Application Laid-Open No. 2003-348041
FIG. 22 shows an operation in which when the time window is applied in the transmission station 100, the GI remover 205 of the reception station 200 removes cyclic prefixes from a received signal and cuts out an effective signal component of each OFDM symbol. In FIG. 22, as in the example shown in FIG. 18, when the received signal is cut out at a general clipping size in the OFDM scheme, i.e., by a number of samples (NFFT-sample period) which is an FFT target in the FFT 206, for a path #2, only an effective signal component (NFFT-sample) of an OFDM symbol n is precisely cut out; however, for a path #1, due to the window function processing, an effective signal component of the OFDM symbol n is distorted at an end (see a hatched portion 300) of the clipping size (NFFT-sample period) and furthermore a signal from an adjacent OFDM symbol (n+1) is introduced in the OFDM symbol n as interference, and thus, the final reception characteristics are degraded.
To avoid such characteristics degradation, the reception station 200 needs to cut out a received signal so as not to include a time window (band limiting) region (e.g., a region of the hatched portion 300). When it is defined in the whole communication system to apply a time window (band limitation) which is common between transmission stations 100, the reception station 200 can easily adjust a clipping size of a received signal, taking into account the common time window.
However, when only an upper limit of adjacent band radiation is defined and application of a common time window is not defined, since it is considered that a transmission station 100 is designed by selecting a method of reducing adjacent band radiation according to a supported transmission rate, constraint of circuit size, etc., under such conditions, it is easily assumed that application of a time window varies depending on transmission station 100 and thus the degradation of reception characteristics, as described above, occurs.